Did you ever have a discussion with someone about Usability on the UI you created? How often did personal preferences play a role rather than objective arguments? Using the technique of quantifying and measuring Usability demonstrated in this video, you might find a new weapon to defend your UI decisions in a scientific way!
Hands down. Both, the “virtual” on-screen keyboard, as well as the “physical” slide-out keyboard aren’t the best input methods out there for mobile phones.